The present invention relates to a feed-forward amplifier for use mainly in the high-frequency band and, more particularly, to a feed-forward amplifier which has a distortion detector for detecting a nonlinear distortion component generated by a main amplifier and a distortion canceller which amplifies the detected distortion component by an auxiliary amplifier and injects it again to the output of the main amplifier, thereby canceling the distortion component.
In FIG. 1 there is depicted the basic configuration of a feed-forward amplifier. The feed-forward amplifier comprises two signal cancellers for compensating for distortions that the main amplifier produces. One of the signal cancellers is a distortion detector 11 and the other a distortion canceller 12. The distortion detector 11 is made up of an input path 8, a main amplifier signal path 13, and a linear signal path 14. The distortion canceller 12 is made of up a main signal path 15, a distortion injection path 16, and an output path 9. The main amplifier signal path 13 is formed by a cascade connection of a variable attenuator 17, a variable phase shifter 18 and a main 20 amplifier 19, whereas the linear signal path 14 is formed by a delay line 28 and a phase inverter 29. In the main signal path 15 there is connected a delay line 21, whereas in the distortion injection path 16 there are connected in cascade a variable attenuator 22, a variable phase shifter 23 and an auxiliary amplifier 24. Reference numerals 25, 26 and 27 denote a power 25 divider, a power combiner/divider and a power combiner, which are simple no-loss power divider and simple power combiners each formed by a transformer circuit, a hybrid circuit or the like.
A description will be given first of the basic operation of the feed-forward amplifier. An input signal to the feed-forward amplifier is divided by the power divider 25 to two signals for input the main amplifier signal path 13 and the linear signal path 14. In this case, the variable attenuator 17 and the variable phase shifter 18 in the main amplifier signal path 13 are adjusted so that the signals on the main amplifier signal path 13 and the linear signal path 14 are equal in amplitude but opposite in phase to each other. The "opposite phase" condition is met by setting an appropriate amount of phase shift between the input and output terminals of the power divider 25 or power combiner/divider 26, or through utilization of a phase inversion in the main amplifier 19. The distortion detector 11 of such a construction detects a difference component between the main amplifier signal path 13 and the linear signal path 14. It is this difference component that is the distortion component the main amplifier 19 generates. On account of this, the above circuit arrangement is commonly referred to as a distortion detector.
The output from the distortion detector 11 is divided by a power/combiner divider 26 to two outputs that are provided to the main signal path 15 and the distortion injection path 16. The input to the main signal path 15 is the sum of the output from the main amplifier signal path 13 and the output from the linear signal path 14. The input to the distortion injection path 16 is the difference between the output from the main amplifier signal path 13 and the output from the linear signal path 14. The variable attenuator 22 and the variable phase shifter 23 in the distortion injection path 16 are adjusted so that the signals on the main signal path 15 and the distortion injection path 15 are equal in amplitude but opposite in phase at the output end of the distortion canceller 12. As the result of this, the distortion components by the main amplifier 19 are injected into the distortion canceller 12 in the opposite-phase but equal-amplitude relation, and hence they can be cancelled.
The above is an ideal distortion compensating operation of the feed-forward amplifier. In practice, however, it is not easy to maintain perfect balance in the respective circuits of the distortion detector 11 and the distortion canceller 12. And even if their initialization is perfect, amplifier characteristics vary with ambient temperature, the power supply and so forth; hence, it is extremely difficult to provide stable and excellent balance over a long period of time.
To maintain the balance of the distortion detector 11 and the distortion canceller 12 of the feed-forward amplifier, there has been proposed an automatic adjustment method using a pilot signal, for example, in Japanese Patent Application Laid-Open Gazette No. 1-198809 entitled "Automatic Adjuster for Feed-forward Amplifier." A device based on such a scheme is described in "Extremely Low-Distortion Multi-Carrier Amplifier for Mobile Communication System--Self-Adjusting Feed-forward Amplifier (SAFF-A)" The Institute of Electronics, Information and Communication Engineers of Japan, Technical Report, RCS90-4, 1990.
In FIG. 2 there is shown in block form an example of the configuration of the feed-forward amplifier using the pilot signals. As depicted, the feed-forward amplifier is provided with: a first pilot injector 32 connected to the input path 8 of the distortion detector 11 to multiplex a first pilot signal PL.sub.1 from a first pilot signal generator 31 onto a transmission signal; a first pilot signal extractor 33 connected between the power combiner/divider 26 and the variable attenuator 22 to extract the pilot signal PL.sub.1 ; a second pilot injector 35 connected between stages of the main amplifier 19 to inject a second pilot signal PL.sub.2 from a second pilot signal generator 34 into the transmission signal; and a second pilot signal extractor 36 connected to the output path 9 of the distortion canceller 12 to detect the second pilot signal PL.sub.2. The levels of the first and second pilot signals PL.sub.1 and PL.sub.2 extracted by the first and second pilot signal extractors 33 and 36 are detected by first and second pilot level detectors 37 and 38, respectively, and the level detected outputs are provided to a controller 39. The controller 39 controls the variable attenuators 17 and 22 and the variable phase shifters 18 and 23. That is, the first and second pilot signals PL.sub.1 and PL.sub.2 are used to detect the balance of the distortion detector 11 and the distortion canceller 12; and the detected balance is appropriately adjusted using the first variable attenuator 17 and the first phase shifter 18 inserted in the main amplifier signal path 13 and the second variable attenuator 22 and the second variable phase shifter 23 inserted in the distortion injection path 16 of the distortion canceller 12. By this, the distortions produced by the main amplifier 19 are compensated for. To achieve the balance of the circuits 11 and 12, the variable attenuators 17 and 22 and the variable phase shifters 18 and 23 are electrically controlled on a stepwise basis to minimize the levels of the pilot signals, for example, by such a simple control algorithm as the perturbation method or steepest descent method or by an adaptive control algorithm based on the least square estimation method. Such automatic control can easily be implemented by a microcomputer.
To process pilot signals in such an automatic adjustment circuit, there has been proposed a simple method using single-frequency pilot signals (for instance, Japanese Pat. Appln. No. 3-49688 entitled "Feed-forward Amplifier"). This method permits simplification of the circuit configuration but involves the necessity for raising the level of the pilot signal to enhance the sensitivity of its detection because the optimum operating point in this method is set at a point of the lowest level of the pilot signal detection. In this instance, if an interference signal such as leakage power of other devices or noise gets mixed into the pilot signal detection band in the feed-forward amplifier, an error arises in the detection level and prevents the feed-forward amplifier from achieving a high precision control operation and the optimum operation.
To implement a feed-forward interference circuit that permits a high precision pilot detection insusceptible to the interference by various kinds of noise, there have been proposed a scheme using a pilot signal modulated at a low frequency (Japanese Patent Application Laid-Open No. 5-9084 entitled "Feed-forward Interference Circuit") and a scheme using a pilot signal obtained by modulating a low-frequency signal through the frequency spread spectrum technique (Japanese Patent Application Laid-Open No. 4-364602 entitled "Feed-forward Interference Circuit"). These schemes are described in U.S. Pat. No. 5,166,634.
The automatic adjustment method of the feed-forward amplifier that uses such pilot signals enables separation of the bands of the pilot signal and the transmission signal; hence it is effective in transmitting amplifiers of FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) radio communication systems.
On the other hand, it is impossible, for the reasons given below, to use the conventional pilot signal intact in the transmitting amplifier for the CDMA (Code Division Multiple Access) radio communication system. First, since the CDMA carrier frequency band is wider than in the traditional TDMA and FDMA radio communication systems, the assignment of one frequency to the pilot signal will seriously reduce the cost efficiency of the radio communication system. Secondly, if the pilot signal is inserted in the transmitting frequency band, it is difficult to make the level of the pilot signal sufficiently lower than the level of the transmission signal and provide a sufficiently high degree of accuracy in the detection by the level detector. The reason for this is that the CDMA transmission signal suppresses the sensitivity to the pilot signal. Thirdly, even if the second problem is solved, the pilot signal interferes with the transmission signal because the pilot signal is not orthogonal to the transmission signal whose output is controlled at all times.
For the reasons given above, the pilot signal generating and detecting schemes for the automatic adjustment of the conventional feed-forward amplifiers are lacking in the practical applicability to the low-distortion transmitting amplifier for the CDMA radio communication system.